What is regrind in injection moulding?

Regrind is sprues, runners and rejected parts from your own production, ground into granules and blended back with virgin resin. It is the cheapest recycled material there is — you already own it — but every extra heat history degrades the polymer slightly, so ratios must be controlled rather than improvised.

How much regrind can you add to injection moulding resin?

Typical limits: 0% for optical, medical and food-contact parts; up to 10% for cosmetic A-surfaces; 15–30% for general engineering parts; 30–50% for hidden structural parts; and up to 100% for non-critical industrial items like dunnage. The right number depends on the resin, the part's mechanical demands and how many times the material has already been heated.

Does regrind weaken plastic parts?

Yes, modestly and measurably. Each heat history reduces impact strength by roughly 5–15% in sensitive resins, shifts melt flow slightly, and shortens glass fibres in filled grades — a 30% glass-filled resin can lose a noticeable share of its reinforcement after several passes. Within controlled ratios the effect is usually acceptable; uncontrolled regrind is where failures come from.

What is the difference between PIR and PCR recycled plastic?

PIR (post-industrial recycled) is factory waste collected before consumer use — clean, traceable and consistent. PCR (post-consumer recycled) comes from used products — variable in colour, contamination and melt flow, but with the stronger environmental claim. PIR suits engineering parts; PCR suits applications where the sustainability story matters and the specification has room for variation.

Does using recycled content save money?

In-house regrind almost always does — it converts scrap you already paid for into sellable parts. Purchased PCR or PIR resin is a different calculation: prices track virgin resin and sometimes carry a premium for certified material, so the business case is usually sustainability targets or customer requirements rather than cost.

Regrind and Recycled Content in Injection Moulding: The Rules

Almost every injection moulding operation generates a stream of sprues, runners, start-up scrap and rejected parts. What happens to that stream — controlled regrind, certified recycled resin, or the bin — affects part quality, material cost and increasingly the OEM's sustainability reporting. This guide sets out the rules: how much regrind different applications tolerate, what repeated heat history does to the polymer, and how to specify recycled content so it survives audits.

Virgin, regrind, PIR and PCR: the four material streams

Terminology matters because specifications are written in it:

Virgin resin: new polymer from the producer, with full datasheet properties and traceability
In-house regrind: your own runners and rejects, granulated and blended back — same grade, known history, but one or more extra heat cycles
PIR (post-industrial recycled): scrap collected from manufacturing before consumer use — clean and consistent, but history outside your control
PCR (post-consumer recycled): material recovered from used products — the strongest environmental claim, the widest property variation

In-house regrind and purchased recycled resin are different tools: the first is a cost lever, the second is a sustainability lever.

How much regrind is safe? It depends on the application

Application	Typical maximum regrind	Reason for the limit
Optical, medical, food-contact	0%	Validation and regulation forbid uncontrolled history
Cosmetic A-surfaces	0–10%	Colour drift, gloss change, black specks
General engineering parts	15–30%	Modest property loss within specification
Hidden structural parts	30–50%	Mechanical margin absorbs degradation
Non-critical industrial (dunnage, spacers)	Up to 100%	Function tolerates variation

These are planning figures, not permissions. The actual limit for your part comes from its mechanical margin and its specification — a snap-fit living on 30% of its strain capacity cannot afford a 15% impact-strength loss, while an over-engineered bracket can. If the part carries a critical feature, the materials guide helps judge how sensitive the chosen resin is to heat history.

What does regrind do to the material?

Every pass through the barrel is a heat history, and polymers keep score:

Impact strength falls roughly 5–15% per additional heat cycle in sensitive resins (ABS, PC, PA); stiffness is barely affected
Melt flow index drifts upward as chains shorten, changing filling behaviour — a process tuned on virgin resin may flash or burn on high-regrind blends
Glass fibres break with each granulation and moulding pass; GF30 regrind is effectively a weaker-filled compound
Colour drifts, usually yellowing, and black specks from degraded material appear — the classic cosmetic complaint, catalogued in the defects troubleshooting guide
Hygroscopic resins must be dried again — regrind absorbs moisture like virgin resin, so the rules in the resin drying guide apply to every blend containing it

None of this forbids regrind; it forbids uncontrolled regrind.

Rules for running regrind well

Granulate promptly and cleanly — runners and rejects go into dedicated, labelled bins by material and colour, never a mixed scrap bin
Control the ratio gravimetrically — a blender dosing by weight at a fixed percentage, not an operator tipping a bucket
Cap the generations — regrind that already contains regrind carries multiple heat histories; many shops allow only first-generation regrind in quality parts
Keep sources known — only your own parts in a known grade; never bought-in or returned scrap of uncertain origin
Record the blend — the regrind percentage is a process parameter and belongs in the batch record, for traceability and for the next audit

Buying recycled resin: PIR and PCR

Purchased recycled resin is a sourcing exercise, not a scrap-handling exercise. Specify the base grade, the recycled content percentage and the acceptable melt-flow window; ask for certification where it exists — EuCertPlast and RecyClass for European recyclers, UL 2809 for recycled-content verification. Expect batch-to-batch variation wider than virgin resin, especially in PCR: colour, MFI and odour all move. Price tracks virgin resin closely and certified grades can cost more, so the business case is normally a customer requirement or a corporate recycled-content target, not savings.

Food-contact and regulated limits

Food-contact use requires compliance evidence for the material and process. EU rules distinguish recycled waste from qualifying production by-products for reprocessing. Medical use requires the approved material specification and validation. Assume no regrind until its use is approved. 10/2011, 2022/1616 & 2025/351. food-contact guide.

What should go into the specification?

A regrind policy that lives in an operator's head fails its first audit. Write down: the maximum permitted regrind percentage per part number, the permitted sources (in-house only, or named certified suppliers), the prohibition list (medical, food-contact, optical, safety-critical), and the change-control rule — any change to the declared ratio is a process change requiring customer notification where a PPAP or quality agreement applies. Agree the permitted regrind sources, maximum blend ratio, batch records and change-control requirements with Nordmould for each part number before production.

Related guides

Need a regrind or recycled-content policy written into your supply agreement? Ask Nordmould for a quotation — Agree the blend ratio and documentation requirements in the project specification.